1. Technical Field
The present invention relates to a tennis racket.
2. Related Art
It is well known that a tennis racket generally includes a frame and a string. The frame has a structure including a looped frame body (hereinafter referred to as a loop portion) that serves as a ball hitting face (hitting face) and a shaft portion provided with a grip at an end thereof. The hitting face is formed by stringing across a string inwardly of the loop portion.
As described in JP-A-6-269514, there are various performances required for the tennis racket and one of the important performances required is a “hitting feel”. The hitting feel is a performance related to a feel that is transmitted to an arm or a hand holding the racket upon hitting the ball and can be quantitatively expressed, for example, by an amplitude of vibration of the string, time taken for the amplitude to damp, or a coefficient of restitution when the racket is supported at a grip position in the air with the hitting face horizontal to the ground surface and a ball is dropped from a predetermined height to the hitting face in such a state.
Qualitatively, the hitting feel is often expressed with an expression such as “hard” and “soft”. Generally, “hard” represents a feel in which there is a strong impact in hitting the ball but a high ball speed can be obtained and the racket can be quickly swung through, and may also be referred to as a “agile feel/speed feel”. On the other hand, with the “soft” feel, even if the ball is hit outside a central region in the hitting face, a so-called sweet spot, which provides an easier repulsion of the ball, the impact is weak. Also, it represents a feel with which a high ball speed cannot be achieved but a ball direction can be positively controlled. This may also be referred to as a “refreshing feel/comfort feel”.
Preference between such feels certainly differs between players depending on their age, sex, skill level and physical strength and also differs for the same player depending on his or her training hours and time, season and physical condition. For example, a player may prefer a hard feel at the beginning of the training since he/she has physical strength, whereas he/she may prefer a soft hitting feel as he/she gets exhausted by the training and a power of the arm to receive the shock from the ball is reduced. Further, a string expands and contracts in accordance with temperature and humidity. Further, a repulsion force of the ball changes due to a change in an elasticity of rubber forming the ball in accordance with the temperature. Therefore, even if the same racket is used, the hitting feel may change through the year.
Whether being a professional or an amateur, there is a wide variety of tennis players. There is a wide range of physical strength and skill for individuals between different age and sex and even between those of the same age and sex. Accordingly, there are various the hitting feels preferred by individual players. Recently, there are many players who wish to play with different hitting feels depending on the physical strength or the mood that occasionally change or depending on the season, etc.
In the past, since the hitting feel relied on a tension of a string or a flexibility of the frame itself of the racket, if the player wanted to play with different hitting feels, he/she needed to posses a plurality of rackets with frames of different hitting feels or rackets strung with different tensions, or, depending on the season, to restring by specifying the tension to obtain a preferred hitting feel.
However, for an ordinary amateur tennis player, it is a great economic burden to possess a plurality of rackets. Further, with a training using rackets of different frames in turn, the player cannot get used to the rackets and, particularly, it takes time for a beginner to improve his/her skill. In a case of having it restrung by specifying the tension, it is difficult to obtain a preferred hitting feel unless he/she is an experienced or an advanced-level player since an experience or a professional knowledge on correlation between the tension and the hitting feel is required.